1. Technical Field
The present disclosure relates in general to a processing system, and more particularly to determining a geometric parameter of an optical spot of a light beam of an optical metrology tool used in the processing system.
2. Background Art
Utilization of optical metrology in semiconductor manufacturing has grown significantly over the past several years. The technology provides capabilities to conduct measurements of a wide variety of critical device parameters, including, e.g., critical dimensions, depths and sidewall angles. Benefits of optical metrology include non-contact measurement capability that can be performed very quickly.
For optical metrology tools to yield reliable measurements, it is necessary that the tools produce well defined light beams, and collect the optical response for analysis, as designed. The size of the optical spot of the produced light beam, which is defined as the area illuminated by the incident light beam, can have a large impact on the measurement values. As a consequence, for example, the sizes of the optical spots of optical metrology tools affects the matching performance of the optical metrology tools.
An optical spot can initially appear to be symmetric. An optical spot may have an area (usually referred to as an “effective spot”) inside the optical spot, which provides most, e.g., 99%, of the information content. An effective spot is very sensitive to the small change in the area it illuminates. Usually, an effective spot comprises a small portion of a total optical spot. For example, the entire optical spot may illuminate an area of 1600 square microns, while the most sensitive area of the optical spot (i.e., the effective spot) may be a 100 square micron area. If the optical spot is symmetric, the effective spot will be located in the center of the optical spot. If the optical spot is not symmetric, then the effective spot will be located off the center of the optical spot. Whether an optical spot of an optical metrology tool is symmetric and the level of symmetry impact the measurement result and thus the matching of the optical metrology tools.
Conventional approaches to optical metrology do not have a satisfactory solution to characterize a geometric parameter of an optical spot and or or a level of symmetry of the optical spot.